In a hearing aid it may occur that a fraction of the sound that that emanates from the receiver of the hearing aid may leak back to the microphone. This sound that leaks back to the hearing aid microphone will then be added to the microphone signal and amplified again. This process may thus be self-perpetuating and may even lead to whistling when the gain of the hearing aid is high. This whistling problem has been known for many years and in the standard literature on hearing aids it is commonly referred to as feedback, ringing, howling or oscillation.
Usually the onset of whistling is dependent on the gain in such a way that whistling will be more likely to occur the higher the gain is. Hence, whistling thus poses a limit on the maximum gain that may be achieved in most hearing aids.
An early approach to solve the problem of whistling was to manually reduce the gain, for example by adjusting a volume control, when the user experienced whistling. However, this solution to the problem of whistling is unsatisfactory, because the whistling is annoying for the user of the hearing aid, and the experience of whistling is usually painful and may even be directly harmful to the individual that experiences it. Therefore, it has been of great importance to develop methods of automatic whistle detection, before or no later than the onset of whistling, while at the same time providing the user of the hearing aid with as much gain as needed in order to compensate for the hearing loss of said user.
In order to achieve a high gain, while at the same time preclude whistling several automatic feedback cancellation, and whistle detection techniques have been employed in the past, one of which is disclosed in U.S. Pat. No. 6,650,124.
U.S. Pat. No. 6,650,124 discloses a method of reducing whistling in hearing aids, where the method comprises the step of evaluating whether a frequency component of an input signal is whistling by calculating the variance of the signal component and comparing it to a threshold. The whistle detection is thus based on a variance criterion. Then if it is determined that the frequency component relates to whistling, then a switch activates a notch filter that filters out the particular frequency. There are however, several drawbacks of this method of whistle detection and whistle suppression. First, the method disclosed in U.S. Pat. No. 6,650,124 does not disclose any efficient way of determining which frequency components of the input signal of the hearing aid needs to be analyzed by the variance criterion (in fact it is not clear how the signal is estimated, since U.S. Pat. No. 6,650,124 is silent with respect to this). Secondly, the application of a variance criterion comprises the calculation of a 2′nd power (a squaring calculation), which is a complicated arithmetical operation that requires much processing power and in addition to this a much wider dynamic range (e.g. when a 16 bit number is squared it becomes a 32 bit number), especially if one consider the limited processing power that is available in present day hearing aids. Thirdly, the proposed method of whistle suppression by the use of a notch filter is very inflexible and since a notch filter simply filters out a given frequency or a very narrow frequency region around a given frequency, the application of a notch filter for whistle suppression may lead to audible changes or distortions of the signal, which may be heard and perceived as annoying for a user. Furthermore, the predetermined width of the notch filter will imply that it in some circumstances will be too wide, while it in other circumstances will be too narrow, and in case of a false detection of whistling the application of the notch filter will lead to a perceptual loss of signal power.